Transmission electron microscopy (TEM) is an effective imaging technique for the direct visualization of structures from the multicellular to the macromolecular size range. The information provided by TEM contributes to the resolution of basic scientific questions as well as to more defined regulatory issues. This year the major contributions of TEM to CBER's mission have been in the areas of: product safety (cell substrates and the presence of adventitious agents), viral pathogenesis (virus-cell interactions, viral replication, cell death), vaccine development (plasmid-derived particles), and assay development (retroviral testing methods). The regulatory research carried out by the EM lab in collaboration with other CBER scientists has served to promote the safety of biologic products, to increase knowledge of viral pathogenesis and to aid in the development of new vaccines.